1. Field of the Invention
The present invention relates to a subsea wellhead assembly for an oil and/or gas well, and more particularly to a tubing hanger assembly and annulus sleeve that allows simultaneous control over both the production casing annulus and casing annulus outside the production casing.
2. Description of Related Art
A typical subsea wellhead assembly includes a wellhead housing installed at the sea floor. Generally, as a well bore is drilled, successive concentric casing strings are installed in the well bore. The area between adjacent casings strings is known as an annulus.
Typically, each successive casing string is cemented at its lower end, and includes a casing hanger sealed with a mechanical seal assembly at its upper end in the wellhead housing. Accordingly, the upper end of each casing string is sealed, and any fluid located in the annulus between adjacent casing strings is thereby trapped. Typically, cement is not brought to the casing shoe at the lower end of each casing string. This allows thermal expansion of the fluid trapped in the annulus of the casings into the formation through the lower end of each casing. The amount of thermal expansion of this trapped fluid, as well as other properties of the fluid (e.g., pressure, etc.), are generally unknown during the drilling and production phases of the well.
A greater knowledge of the properties of the fluid trapped in the annuluses of the casing strings would be of great benefit during long term production of the well. Specifically, the area within the innermost casing string, known as the production casing, and outside of the production tubing is the “A” annulus. The area immediately outside the production casing, but within the first intermediate casing, is the “B” annulus. For example, at times, when it is anticipated that the well will be produced, cement is placed in the lower part of the first intermediate casing string to seal annulus “B”. This sealing of annulus “B” helps to protect the formation from leakage in the event that fluid gets into annulus “B” from the production reservoir. One downside to sealing annulus “B”, however, is that high pressure areas may develop once it is sealed. While it is known to use burst or rupture disks to relieve pressure and reduce the possibility that the production casing will collapse, this can result in leakage into the environment. It would be preferable to be able to monitor and control the pressure inside annulus “B”. In addition, the casings that are installed deep in the well may be less pressure-resistant, thereby increasing the possibility of collapse if the pressures in the annuluses become too great. The ability to monitor and control the pressures in both annulus “A” and annulus “B” could prevent such leakage of the fluid or collapse of the production tubing and/or production casing.
When a well has been drilled and cased, certain steps are taken to prepare the well for production. For example, a production tubing string and a tubing hanger are run into the well bore through the BOP stack. Typically, the tubing hanger is landed, sealed, and locked in the wellhead housing and/or the production casing hanger. Thereafter, the production bore extending through the tubing hanger is sealed, the BOP stack is removed, and a christmas tree is lowered onto the wellhead housing. A christmas tree is an assembly of valves, spools, pressure gauges and chokes fitted to the wellhead of a completed well to control production. It is important to the operation and safety of the well that that the proper connections are made between the christmas tree, the wellhead housing, and the tubing hanger. While most current christmas tree designs provide for measuring and controlling pressure in annulus “A”, none are capable of measuring and/or controlling pressure in annulus “B”.
Accordingly, it would be desirable to have a subsea well completion system that allows for monitoring and control of pressure in both the “A” annulus and the “B” annulus. In fact, a device that provides the ability to monitor and control pressure in the “B” annulus is something that has been asked for by regulatory bodies. However, it has heretofore been given regulatory exemption because it does not exist.